This invention relates to a process for the surface treatment of structural parts, working parts, or the like, made from a ceramic material, which are to be exposed to relatively high temperatures.
An example of the use of a ceramic material at high temperatures is in a gas turbine made of a ceramic material, and whose component parts such as blades, distributor, combustion chamber, nozzle cone, etc. are exposed to temperatures in the range of 1400.degree. C. when in operation.
It has now been established that ceramic materials, such as silicon nitride (Si.sub.3 N.sub.4) or silicon carbide (SiC), oxidize on the surface at temperatures starting from about 800.degree. C., thus forming a glass layer which, depending on the chemical composition, slows down further oxidation because it has a retarding effect on the supply of oxygen to the base material thereunder. The aforementioned chemical composition of the surface glass layer is essentially determined by impurities in and/or additions to the ceramic base material.
In the case of a porous ceramic material, e.g., reaction-sintered silicon nitride, further oxidation produces particularly unfavorable results, since the oxygen has free access to the base material through the pores, so that the material is oxidized to a very large extent, or even entirely, and thus becomes useless.
Efforts have been made to improve the resistance to oxidation of ceramic parts by avoiding, during production and to the extent possible, the inclusion of impurities in the base material. However, thus far such measures have not led to any satisfactory results. Moreover, the use of sintering auxiliary agents, such as calcium, is of fundamental importance to the production of ceramic materials; however, use of such agents causes the resistance to oxidation to be substantially reduced.